I've used parma, pactrica and spaz stix.. i love spaz stix.. but the price sux, parma i have no chipping pealing issues like i have with oil based though, bu ti also backed it with il based paint so it was sealed, the paint is cheap if you shop around, clean up is cake, and making custon colors is easy due to buying 20$ worth of 2.50 each bottles of paint.
pactrica does have a few goodies to it, like gold metalflake, candy blue, red, purpal.. and of corse i use their silver, white, black to back stuff with costs less than other things i can get local, but thats how i do custom colored tinted windows..
i even built up candy green one day spaz stix and pactrica paints for a parma fire truck body i did.. with golf metal flake.. so i say learn them all.. play with them all, each will assist the other in some manner if your creative ehough..